low hot water pressure

Hello,
I have a Valley Old Style(single handle)faucet in tub. I have gotten a few replacement parts for it in the past. Now, when I turn on the water, the hot water pressure is very low in both the tub spigot and the shower head. The cold water pressure is fine. Pressure is fine everywhere else throughout the house. I assume the problem is somewhere in the fixture. I redid the bathroom a few years back and choose(wrongly)not to replace the faucet. I really hate the thought of cutting into my tile and cement board job to replace the whole faucet. Any suggestions would be appreciated. thanks.